This invention relates to a unitary superconducting electromagnet having an outer ferromagnetic body about an inner body containing coils and being especially adapted for use in medical or test apparatus with nuclear magnetic resonance (NMR) and spectroscopy.
Heretofore, as shown in U.S. Pat. No. 4,646,045, issued Feb. 24, 1987, ferromagnetic shields have been positioned about an electromagnet used in medical apparatus, such as used in magnetic resonance imaging (MRI), for example. Such a ferromagnetic shield reduces the magnetic stray or fringe field but has an undesirable effect on the magnetic field homogeneity or uniformity. It is noted that magnetic iron saturates at a magnetic field intensity of around one and one-half (11/2) Tesla and electromagnets requiring a high precision magnetic field, such as one part per million, have not been utilized generally heretofore at a magnetic field intensity about around two (2) Tesla, particularly where iron shields have been utilized since it is well known that magnetic iron shields at an intensity above around two (2) Tesla have had undesirable effect on the magnetic field homogeneity or uniformity thereby restricting the uniformity of the magnetic field. It is noted that the shielded electromagnet set forth in U.S. Pat. No. 4,646,045 has a uniformity of only around two (2) parts per million at a magnetic intensity of the saturation of iron.
At times, it is desirable to have a portable NMR magnet which can be utilized in portable magnetic resonance imaging apparatus, for example, and a ferromagnetic shield would be required for such a portable magnet for containing the magnetic fringe field. The ferromagnetic shield provides a closed magnetic path and affects the shape and intensity of the magnetic field. Heretofore, a ferromagnetic shield for a superconducting electromagnet has not been provided in which the positioning of the coils and ferromagnetic shield in relation to each and in relation to the magnetic field could be predetermined so that a magnetic field uniformity of at least one (1) part per million is obtained, particularly when utilized in a magnetic field having a intensity greater than the magnetic saturation of the ferromagnetic material such as iron, which saturates at 1.5 Tesla. When superconducting electromagnets are utilized in spectroscopy a very high magnetic field uniformity or homogeneity is required, such as one (1) part in one hundred million and also a very high magnetic field intensity is required, such as eight (8) Tesla, for example. Thus, the requirements for a ferromagnetic shield about a superconducting electromagnet for spectroscopy are even more precise than those utilized for magnetic resonance tomography used in research or in magnetic resonance imaging.